A majority of solid oxide fuel cell developers are pursuing planar cell geometry with an anode supported cell design (ASC) and metal interconnects. The major challenges of the ASC technology are: cell fabrication and operational reliability; cell electrical contact to the interconnect; and sealing of the cell-to-metal interconnect. The thin electrolyte, normally 10-15 microns thick, is supported on a relatively thick anode made of nickel oxide and yttria stabilized zirconia (NiO—YSZ), which is a cermet having a thickness on the order of 700 to 1,000 microns.
The anode/electrolyte bi-layer is sintered first, followed by application of a thin cathode, usually 25-50 microns thick, which is fired at a lower temperature than the ASC to create a complete, ASC, solid oxide fuel cell (SOFC). Such cells are arrayed in stacks wherein the individual cells are in series electrical contact with one another by means of metal interconnects.
The difficulties in ASC cell and stack fabrication and operation include: 1) shrinkage matching of the thick NiO—YSZ cermet anode and the thin YSZ electrolyte layers during the sintering process; 2) production of flat cell parts for assembly into fuel cell stacks; 3) as the nickel component of the anode cermet is reduced from NiO to Ni metal, the resultant volume change can generate stresses within the anode, sometimes leading to failure of the thin YSZ electrolyte; 4) the anode is sensitive to leaks of oxygen that can cause oxidation of the Ni metal resulting in a sudden expansion of the anode and failure of the cell; 5) provision of sufficient anode thickness to support the electrolyte creates a long pathway for reactant and can lead to diffusion problems in the anode making it hard to achieve high fuel utilization required for high-power commercial applications; 6) ASC cells are fragile and cannot tolerate the high compressive loading that is required for some of the compression type seals used with the ASC stacking technology; 7) pore channels of the bulk anode must be relatively narrow in order to give the anode adequate strength, but this limits the rate of gas diffusion into the interior of the thick anode and limits fuel utilization; and 8) the grooved metal interconnects are on the order of 2-3 mm thick and contribute more than 70% of the mass of a stack.
The most critical technical challenges facing all planar SOFC stack developers are the difficulties in providing adequate electrical contact between each cell and its metal interconnect and gas-tight seals. For example, a typical anode supported cell, 7 centimeters in diameter operating at 750° C. has a power density of 0.9 W/cm2; if that same cell is now placed between two metal interconnects, the performance now drops to 0.6 W/cm2, a full 33% loss of the power, due simply to electrical contact resistance. Electrical contact resistance, gas sealing, and ease of fabrication are at the center of the new SOFC design.